The invention relates to a suspension system, and more particularly, to an active seat suspension with a hydraulic actuator in parallel with a pneumatic air spring or air bag.
Passive suspension systems, such as for a vehicle seat, are known. For example, John Deere production 6000, 7000, 8000 and 9000 Series tractors have passive seat suspension systems which include a hydraulic shock absorber in parallel with an air bag. Active suspension systems are known which include an electro-hydraulically controlled actuator working in parallel with a resilient device, such as a spring. For example, U.S. Pat. No. 4,363,377 (Van Gerpen), issued Dec. 14, 1982, discloses an active seat suspension system with a hydraulic actuator in parallel with a spring. A control system controls fluid communication to the actuator in response to a seat position signal, a stiffness control, a seat height control and a gain control. U.S. Pat. No. 6,000,703 (Schubert et al.), issued Dec. 14, 1999, discloses an active cab or seat suspension control system with a hydraulic actuator in parallel with a pneumatic air spring or air bag. An active seat suspension system which actively controls the seat isolation with hydraulics and an accelerometer is shown in An Active Seat Suspension System For Off-Road Vehicles, by Grimm, et al. The function of the air bag is to take load off of (xe2x80x9coffloadxe2x80x9d) the hydraulic actuator by supporting the suspended mass. The hydraulic actuator is actively controlled to dynamically isolate the seat from the base upon which it is mounted. Such a system is desirable because, by having the static weight suspended by an air bag, the forces and pressures on the actuator and its hydraulic system are reduced, thus reducing the overall power required for active control and isolation. When the air bag xe2x80x9coffloadsxe2x80x9d the hydraulic actuator during static conditions, the forces required from the hydraulic actuator would essentially be zero.
Typically, in such systems, the suspended mass of the system changes due to changes in operator weight, or changes in vehicle ballast. Weight changes effect the equilibrium position of the suspension, and in an active suspension with offload and closed loop position control, it is desirable to maintain the control position set point at the equilibrium position of the suspension. Therefore, it is desirable to have a control system which automatically adjusts the offload equilibrium position (via and air bag) to match a control position set point, or to adjust the control position set point to match the offload equilibrium position. In a system with an air offload device, such as a compressible air spring or air bag, the offload force which effects the equilibrium position can be adjusted by increasing or decreasing the amount of air in the air spring using an electro-pneumatic compressor and an electronic vent valve. Whether or not the offload force or position set point must be adjusted can be determined by sensing the hydraulic pressure in the actuator. But, using pressure sensors can be expensive and complicated. Accordingly, it would be desirable to have a means for adjusting the offload or position setpoint which does not require pressure sensors.
Accordingly, an object of this invention is to provide an active suspension system with an actively controlled actuator and an offload device, wherein the offload and/or position setpoint can be adjusted without pressure sensors.
This and other objects are achieved by the present invention, wherein an active suspension system for supporting a mass, such as a seat on base of a vehicle, includes a hydraulic actuator coupled between the seat and the base, an pneumatic off-load device between the seat and the base, and a control system which actively controls the hydraulic actuator and which controls the off-load member. The control system actively controls the hydraulic actuator as a function of a seat position error signal, and the seat acceleration signal. The control system also controls the off-load member as a function of the seat position error signal. A compressor and a vent are coupled to the pneumatic device for controlling pressurization thereof. The control system comprises an electronic control unit which repeatedly executes a control algorithm at a predetermined rate, and which controls the operational status of the compressor and the vent as a function of the position error signal and the predetermined rate.